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Hello, Is there a mutex implementation available for use that does not require the rtx lib? Thanks, JG
Andrew, Thanks for the reply.
I cannot disable the interrupts.
I had read both references you provide.
I am not an assembly programmer so regarding the first reference, there is no complimentary "unlock" function, and no example of how to invoke the lock (I assume I must pass a volatile int as an arg).
Regarding the 2nd reference, the last line of text is "However, it does not work".
// interrupt scannot wait, sleep for meddle with the task scheduler as they must be as fast as possible. // the function refuse_interrupt_context() invokes a software error if this (or other) systems calls is called from // interrupt context int32s rtos_mutex_lock(int32s a_id) { int32u l_scheduler_state ; int32s l_result = NO_ERROR ; refuse_interrupt_context(__FILE__, __LINE__) ; // first check if the mutex was allocated. if ( (a_id >=0) && (a_id < MAX_MUTEXES) ) { t_mutex *lp_mutex ; tcb_t *lp_task ; // the kernel is not reentrant; protect static/global data from ISRs scheduler_disable(&l_scheduler_state) ; lp_task = *(g_tcb + g_current_task_index) ; lp_mutex = s_mutexes + a_id ; // loop here and reschedule if a lock attempt fails while (lp_mutex->owner != g_current_task_index) { // attempt to lock the mutex if (lp_mutex->state == e_mutex_unlocked) // note: the lock is never relinquished if there is a task waiting for the mutex in order to prevent a race condition // mutexes (another mutex might lock it before the dequeued task (from the mutex's queue) gets a chance to lock it. after the next owner of the lock is // scehduled (because it is being put in the running queue), it is restarted at this function (after the call to 'scheduler_reschedule', see below - that is // the new return address of the task) and locks the mutex. a mutex is freed only is there is no task waiting for it. { lp_mutex->owner = g_current_task_index ; lp_mutex->state = e_mutex_locked ; ++lp_task->mutex_lock_reference_counters[a_id] ; // increment the refrence counter. a task that has locked a mutex 'x' times must release it 'x' times } else if ( (lp_mutex->state == e_mutex_locked) && (lp_mutex->owner == g_current_task_index) ) // check if the running task attempts to lock a mutex that it already owns { // task is already owner - nothing to do ++lp_task->mutex_lock_reference_counters[a_id] ; // increment the refrence counter. a task that has locked a mutex 'x' times must release it 'x' times } else // if the mutex is locked, and the calling task is not the owner, it will have to wait { int32s l_result ; // move task to the waiting tasks queue of the mutex. this task does not get any CPU time unless moved back to the running queue. // remember that g_current_task_index is absent from the running queue (because it was dequeued) if ( (l_result = priority_queue_insert(&lp_mutex->blocked_tasks, (*(g_tcb + g_current_task_index))->priority, g_current_task_index)) != NO_ERROR) { software_error("%d %s %d", l_result, __FILE__, __LINE__) ; } lp_task->status = e_task_blocked_mutex ; lp_task->blocked_on_primitive = a_id ; scheduler_restore(e_scheduler_enabled) ; scheduler_reschedule() ; scheduler_disable(&l_scheduler_state) ; // after a lock attempt has failed, execution resumes here // which means the scheduler is enabled again } } scheduler_restore(l_scheduler_state) ; } else // report an error { software_warning("%d %s %d (%d)\n", ERR_MUTEX_NOT_FOUND, __FILE__, __LINE__, a_id ) ; l_result = ERR_MUTEX_NOT_FOUND ; } return l_result ; }
// never use from within interrupt context. see comments for rtos_mutex_lock. int32s rtos_mutex_unlock(int32s a_id) { refuse_interrupt_context(__FILE__, __LINE__) ; if ( (a_id >=0) && (a_id < MAX_MUTEXES) ) { int32u l_scheduler_state ; t_mutex *lp_mutex ; tcb_t *lp_task ; // the kernel is not reentrant; protect static/global data from ISRs scheduler_disable(&l_scheduler_state) ; lp_task = *(g_tcb + g_current_task_index) ; lp_mutex = s_mutexes + a_id ; if (lp_task->mutex_lock_reference_counters[a_id] > 0) { lp_task->mutex_lock_reference_counters[a_id]-- ; // always decrement the reference counter, no matter what task unlocks the mutex } // if the mutex is locked by the calling task and that task has reliqueshed all its // referneces to the mutex (a task call lock a mutex multiple times if it is the owner) // - allow other tasks try to lock it if ( (lp_mutex->state == e_mutex_locked) && (lp_mutex->owner == g_current_task_index) && (lp_task->mutex_lock_reference_counters[a_id] == 0) ) { int32s l_unblocked_task ; // select a task to be returned to the running queue, so that the scheduler can select it // there is at least one task blocked on the mutex - prepare it to be scheduled lp_mutex->owner = ERR_MUTEX_NOT_FOUND ; lp_mutex->state = e_mutex_unlocked ; if (priority_queue_extract_minimum_data(&lp_mutex->blocked_tasks, &l_unblocked_task ) != ERR_QUEUE_EMPTY) { // WAR STORY // this is the fairest approach: if a task has been waiting and the lock has become available, // select the next task from the blocked queue (note: this is not yet a priority queue) and put it in // the right slot in the ready list, for later selection. Howver, this introduces a problem of // possible starvation - the task needs the lock again but it might be lock by another task once it is // scheduled again. different approaches, such as granting the CPU to the extracted task in // the next task switch are doomed to fail, because they might introduce a priority inversion: if the // unblocked task task was a low priority task that plans to keep the lock for a while, it might be // preempted by a high priority task which will have to wait. //tcb_t *x = *g_tcb + l_unblocked_task ; scheduler_declare_task_ready(l_unblocked_task, g_tcb[l_unblocked_task]->priority) ; scheduler_restore(e_scheduler_enabled) ; scheduler_reschedule() ; } } scheduler_restore(l_scheduler_state) ; } else { software_warning("%d %s %d\n", ERR_MUTEX_NOT_FOUND, __FILE__, __LINE__) ; } return 0 ; }
typedef enum { e_mutex_unlocked = 0, e_mutex_locked } mutex_state ; typedef struct { mutex_state state ; // the status of the synchronization element int32s owner ; // a mutex has one owning task. other tasks must wait for it to be released priority_queue_t blocked_tasks ; // // a queue of task id's that are waiting for a mutex to be released } t_mutex ;
static t_mutex s_mutexes[MAX_MUTEXES] ; // system mutexs
You would, of course, need your _own_ kernel to use anything like this...
Thanks for the code see!